As pick boxes are secured to rotary cutting heads by welding and as welding, to replace a worn or damaged pick box, cannot normally be effected in a UK coal mine due to safety regulations, a knock-in, knock-out liner sleeve is frequently used to obviate damage to the pick box, in service--with a worn or damaged sleeve being replaced instead of a worn or damaged pick box.
Known liner sleeves conventionally include at an "outer" end an enlarged head having a seating surface, to seat on an annular seating surface of the pick box. Some sleeves have been intended to rotate, in service, with a view to obtaining even wear on the sleeve and in particular on its enlarged head, which system involves deliberately manufacturing clearances into the components to permit rotation, but the play resulting from such a relatively loose fit causes fretting (progressive enlargement of the play due to metal deformation) in service which in turn results in premature wear between the sleeve and the box aperture. Other proposals have been for a non-rotatable, press-fit sleeve, but impactions sustained in service, coal etc., fines ingress, and rusting, make it difficult, if not impossible, to remove a press-fit sleeve in confined mine conditions, so that worn sleeves often remain in service, resulting in box wear rather than box protection.
Other proposals for non-rotatable sleeves are disclosed in U.S. Pat. No. 5,106,166.
Also, a pick box, into which the sleeve is releasably fitted, is usually provided with a receiving aperture for a water spray nozzle, to discharge a cone etc., of water vapour in the vicinity of the tip of the pick. With a view to protecting the nozzle from damage, so-called rear entry spray nozzles are widely employed, but the location of some such sprays compromises the ability to direct the spray cone in the optimally required location, due to interference of ancillary components, particularly the conventionally provided enlarged head of the sleeve.